Process for cleaning a photoconductive drum of residual toner particles and reuse of the same

ABSTRACT

A METHOD FOR PRODUCING XEROGRAPHIC COPIES IN WHICH A PHOTOCONDUCTOR IS MOVED IN CONTACT WITH A QUANTITY OF TWO COMPONENT DEVELOPER THROUGH AN ACTIVE DEVELOPMENT ZONE TO CREATE A FLOW PATTERN IN THE DEVELOPER MATERIAL, THE IMPROVEMENT BEING CHARACTERIZED BY REMOVING THE RESIDUAL TONER PARTICLES AT THE BEGINNING OF THE DEVELOPMENT ZONE FOR REUSE IN THE FLOW PATTERN.

- 1971 w. c. EMERSON 3,625,683

PROCESS FOR CLEANING A PHOTO-CONDUCTIVE DRUM OI" RESIDUAL TONER PARTICLES AND REUSE OF THE SAME Filed D60. 1, 1969 FIG. I

PRIOR ART DEVELOP- MENT ZONE ATTORAEY United States Patent Oifice 3,625,683 Patented Dec. 7, 1971 3,625,683 PROCESS FOR CLEANING A PHOTOCONDUCTIVE DRUM F RESIDUAL TONER PARTICLES AND REUSE OF THE SAME William C. Emerson, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y. Filed Dec. 1, 1969, Ser. No. 881,043 Int. Cl. G03g 13/14; B05b 5/02 US. Cl. 961.4 2 Claims ABSTRACT OF THE DISCLOSURE A method for producing xerographic copies in which a photoconductor is moved in contact with a quantity of two component developer through an active development zone to create a flow pattern in the developer material, the improvement being characterized by removing the residual toner particles at the beginning of the development zone for reuse in the flow pattern.

This invention relates in general to xerography, and, in particular, to an improved method for producing xerographic copies.

In the art of xerography, as originally disclosed by Carlson in UJS. 2,297,691, a plate, comprising a conductive backing upon which is placed a photoconductive insulating material, is charged uniformly and the photoconductive surface exposed to a light image of an original object to be reproduced. The photoconductive coating is caused to become conductive under the influence of the light image so as to selectively dissipate the electrostatic charge found thereon to produce what is known as a latent electrostatic image. This latent image is then developed by means of a variety of pigmented resins which have been developed for this purpose. The pigmented resin material, or as herein referred to, toner, is electrostatically attracted to the latent image on a photoconductive surface in proportion to the amount of charge found thereon. That is, a small charge concentration becomes an area of low toner density while areas of greater charge concentration become proportionally more dense. The developed image is then transferred to -a final support material and fixed thereto to form a permanent record of the original object.

The most widely employed method of developing a Xerographic image is cascade development. In cascade development, a two-component developer comprising finely divided particles of pigmented resins and larger carrier material which is adapted to act as a vehicle for the pigmented powder, is used to develop the latent electrostatic image. The carrier material, being larger in size relative to the toner material, supports a relatively large number of toner particles on its surface. By preselecting material, a triboelectric attraction between toner and carrier is produced when the materials are brought into contact. Toner is placed in the image areas by cascading toner loaded carrier beads over the plate surface. The charge in the image areas is controlled to attract sufiicient toner material from the carrier so as to develop the latent image. U.S. Pats. to Wallcup et al., 2,573,881; Eichler, 2,965,868; and Carlson 2,990,278, all illustrate the well known cascade method of development.

Although cascade development has proven to be a highly successful method of developing a latent xerographic image, there are nevertheless certain drawbacks associated with the process. The need for relatively large conveyoring means for moving developer material adds greatly to the size of the xerographic apparatus. A powder cloud, which produces unwanted background development, results from the violent cascade action. Plate abrasion and carrier failure are further results attributed to the cascade development system.

In order to overcome the heretofore mentioned disadvantages associated with conventional cascade development, a new method has been devised in which a latent eletrostatic image is developed by bringing a moving photoconductive plate into contact with a two component developer material contained within a developer housing. The plate surface moves in opposition to the force components of gravity acting on the granular developer material so as to establish a developer flow pattern within the housing. The flow provides the motion needed to mix toner and carrier material and to obtain the needed triboelectrification. This type of system is disclosed in copending applications to Gundlach, Ser. No. 528,846, filed Feb. 21, 1966, now Pat. No. 3,503,776 and to Chawda, Ser. No. 697,083, filed Jan. 11, 1968.

Many geometric configurations are known and referred to in the Gundlach patent. All of these systems, except for minor variations, employ the same basic development principle and a reasonable correlation can be made therebetween. This general type of development system has come to be known as C-shell development because a majority of the work performed thereon involves a system having a 'C-shell-shaped developer housing. The tern-r C-shell will be used herein to refer to this type of development system, however, it should be clear that this term is in no way limited to its literal or geometric description.

Although the basic C-shell system overcomes the previously mentioned disadvantages associated with cascade development, C-shell, in its basic form, also has certain inherent disadvantages. Most of these disadvantages are a result of a relatively slow flow rate associated with the basic C-shell system. Insufficient toner and carrier mixing, poor triboelectrification, difliculty in adding new toner to the system, and insufficient delivery of developer material to the active development zone are some of the problems directly attributed to the slow flow rate.

In accordance with the present invention, there is provided an improved reproduction system by greatly enhancing the interaction between carrier and toner material to achieve a greater flow rate with better developer mixing. More than this, the cleaning up of residual toner images is accomplished in the development zone to greatly improve the efficiency of the system.

It is therefore a primary object of this invention to improve xerographic reproduction.

Another object of this invention is to improve basic C-shell development.

It is yet another object of this invention to increase the flow rate in C-shell development systems.

Yet another object of this invention is to increase toner availability in the active development zone of a C-shell development system.

Still another object of this invention is to improve toner distribution and mixing throughout a C-shell development system.

Another object of this invention is to clean residual toner particles for reuse in a C-shell development system.

Another object of this invention is to improve the efiiciency in an automatic xerographic apparatus.

Another object of this invention is to reduce the amount of time required to bring a C-shell development system to optimum operating conditions.

These and other objects of the present invention are attained by brushing the residual toner particles remaining on the photoconductive surface in a developer housing having an active development zone therein in which the photoconductive surface is moved in contact with a two component developer material, the flow in the development zone being supplied with a continual stream of developer material by means of the brushing action in the vicinity of carrier beads at the start of the active development zone whereby the toner particles are reused in the system.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following description of the invention to be read in connection with the drawing, wherein! FIG. 1 is a illustration of the basic C-shell configuraiton found in the prior art; and

FIG. 2 illustrates schematically a preferred embodiment of the xerographic apparatus of the instant invention adapted for continuous use in an automatic xerographic machine; and

FIG. 3 is an enlarged view of certain details of the development apparatus according to the present invention.

A brief discussion of basic C-shell development and the flow mechanism associated therewith is believed warranted at this time in order to more fully understand the teachings of the present invention. In the basic C- shell system as shown in *FIG. 1, drum is rotatably mounted so as to move in the direction indicated through a clam shell-shaped housing 11. The housing contains sufiicient developer material so that the drum contacts the developer material as it moves through the housing to establish a flow pattern therein. This flow of material is depicted by the arrows associated therewith. A thin layer of developer material adjacent to the drum surface moves in an uphill direction at approximately drum speed while the entire back layer of developer (the back side of the system) moves down more or less as a unit to fill the void left by the upwardly moving material.

This up-hill flow of developer material in contact with the drum surface defines what is herein referred to as the active development zone. Theoretically, properly charged and toner loaded developer material is delivered to the start of the active development zone from the supply of developer material found on the backside of the system, The developer material introduced into the active zone is carried along in contact with the drum surface as the drum rotates where development takes place by means of the classical xerographic development-scavenging mechanism. A point is finally reached where the moving drum surface can no longer overcome the forces of gravity acting on the carrier material and the developer material falls away and returns to the backside of the system to fill the void left by the continuous flow of upwardly moving material. As can be seen, the carrier beads leaving the active development zone have given up their toner in the development process and therefore must be replenished on the backside of the system before they can once again be returned to the start of the active development zone.

It has been found that the flow rate in the basic C-shell system is not limited by the drum/developer interface motion but rather by the time required to return replenished developer material to the active development through the backside of the system. Because this is a closed system, the volume rate of flow through the active development zone is equal to the rate of flow found on the backside of the system. In fact, tests have shown that by providing an infinite supply of developer material to the start of the active development zone, a flow rate approximately eight times greater than found in the basic C-shell system can be attained.

From the prior discussion, it should be clear that the backside of the developer system should have the steep est return geometry possible in order to provide the more rapid flow rate through the system. However, the angle of repose of the developer material acts to limit the reutrn geometry of the system. The angle of repose is the maximum angle with the horizontal at which a quantity of loose particulate material will retain its position before sliding. The angle of repose of the developer material within the developer housing 12 precludes the developer from contacting the drum surface for a considerable distance as the drum rotates through the housing. In fact, for most commercially available developer materials, contact is not made until the drum has rotated through approximately 45 from the vertical. The material angle of repose severely limits the lengths of the active development zone and severely restricts the return geometry of the system.

It should be noted, however, that basic C-shell development is nevertheless a highly efiicient system. With a volumetric flow rate somewhere in the nature of between 200 and 400 times less than the flow rate found in most commercially available development systems, the basic C-shell system is capable of producing good copy. This high efficiency is evidenced by the extremely starved condition of the carrier beads as they leave the active development zone.

The present invention is for improving the basic C-shell development and xerographic reproduction system as well as eliminating the difiiculties associated therewith as shown in FIGS. 2 and 3. Although the instant invention is well adapted for use in any suitable xerographic reproduction apparatus, it is shown herein embodied in a drum type xerographic apparatus for purposes of illustration. A drum 10 is mounted on shaft 11 and the shaft rotatably supported in the side frames of the machine (not shown). The major xerographic processing components are mounted around the drum periphery so that they are able to act thereon as the drum continually rotates through the various stations.

In general, the several xerographic processing stations in the path of movement of the drum surface may be described functionally as follows:

a chargingstation A, at which a uniform electrostatic charge is deposited on the photoconductive drum surface;

an exposure station B at which a light or radiation pattern of a copy to be reproduced is projected onto the xerographic drum to dissipate the charge in the exposed areas to form a latent electrostatic image thereon of the copy to be reproduced;

a development station C, at which a xerographic developing material, including toner particles having an electrostatic charge opposite to that of the electrostatic image, are placed in contact with the moving drum surface whereby the toner particles are caused to adhere to the electrostatic latent image found thereon; and

a transfer station D Where the developed electrostatic image is transferred from the plate surface to a final support material.

In accordance with the present invention after transfer of the image to the support material, the drum surface advances past a negative charging apparatus 18 wherein the charge on the drum is reversed in preparation for cleaning as will become more apparent. The drum surface supporting the residual image continues to advance through a discharge station 19 where the entire surface is flooded withlight to discharge the photoconductive insulating layer. After discharge, the surface of the drum is then ready to be charged and exposed again, although the residual image from the previous exposure still remains on the same area of said surface. {In the existing xerographic systems, the drum surface would typically remove the residual image prior to performing the charging1 and exposing steps of the subsequent xerographic cyc e.

The development apparatus of the instant invention comprises a housing, generally designated 20, in which the upper portion of the housing is open to receive a moving photoconductive surface. Housing 20 runs longitudinally the length of the xerographic drum. An area between the lower housing and the drum surface, numerically referred to as 22, defines the active development zone of the development system.

The remaining portion of housing 20 is shaped to form two sump areas. Sump 23 is located upstream from the active development zone and is herein referred to as the upstream sump area. =Likewise, sump 24, which is located downstream from the active development zone, is herein referred to as the downstream sump area. Both sumps are elongated laterally along the length 'of the drum surface and are adapted to communicate with the active development zone along the length of the drum.

Downstream sump 23 has positioned therein one or more rotatable brush members 25 running longitudinal to and in light contact with the drum surface. After the brush bristles 26 wipe the drum surface clean, they are compressed by a flicking member 27 to detach the residual toner particles. Brush bristles 26 may be made out of any suitable material. Typical materials are Dynel, nylon, and various furs, such as, New Zealand rabbit fur, and combinations thereof. Brush member 25 is rotated at speeds ranging from up to about 1.5 to times the speed of the drum in order to ensure removal of the residual toner particles. The shaft portion of brush member 25 extends external to the housing and is driven in a clockwise direction by any suitable drive means, as for example, drive motor 50.

In operation when the system is charged with developer material, drive motor 50 produces a clockwise rotation of brush member 25 against flicking member 27 which, in turn, causes a continuous flow of developer material to be delivered into sump area 23. By this arrangement, the residual images are physically wiped from the drum surface and introduced into the active developer flow adjacent sump area 23 where they are electrostatically attracted to toner-free portions of carrier.

It has been found that a continuous flow of developer material cannot be maintained. An added driving force, beyond the force provided the moving drum surface, is required to move this material into active development zone 22.

As residual toner particles are delivered into the sump area, this added force is suflicient to effect a continuous, trouble-free flow of material from sump 23 into the active development zone 22. In this manner, the apparatus of this preferred embodiment has the capability of producing flow rates far in excess to the flow rates found in basic C-shell development systems.

The form of housing adjacent to the moving drum surface 10 establishes an uphill flow pattern within the housing whereby loaded toner carrier beads are carried in contact with the drum surface towards upstream sump 24. Developer material completely fills the active develop ment zone and rotating drum 10, which is in intimate contact with this material, carries the developer material towards sump area 24. As the loaded carrier beads flow upstream towards sump 24, they give up their toner by means of classic development-scavenging mechanism to develop the latent image in the same manner as a basic C-shell system.

The starved developer material is returned to the sump 23 by flow down the backside of the system which may be facilitated by a rotating auger 35. By this arrangement, starved toner particles fall under the force of gravity into the lower end of the reservoir-toward brush member whereby the cycle is continuous. New toner needed to load the starved carrier material may be readily charged into the system through a suitable opening (not shown).

The brush member 25 is placed at a ground potential. Thus, the grounded brush member bleeds off any positive space charge which might build up on the carrier material, a condition which would otherwise disrupt the triboelectric balance of the system.

As can be seen, flow rates in the instant invention are no longer dependent on the backside geometry of the system but rather upon the speed at which the brush member 25 delivers material to the active development zone.

That is to say, the brush mechanism which continuously introduces residual images for reuse appears, in effect, as an infinite source of developer material to the active development zone. Although the system is not a particularly violent system, in a cascade development sense, the rotary brush cleaning and development arrangement produces more than suflicient interaction between carrier and toner material to insure proper triboelectrification and material mixing without danger to the photoreceptor surface or carrier material.

The cleaning and development arrangement, as herein disclosed, enhances the active development zone from the return geometry of the system thus giving the apparatus greater flexibility than afforded by the basic C- shell system. The angle of repose of the developer material no longer limits the length of the development zone. With the present apparatus, the length of the active development zone is capable of being lengthened or shortened so as to optimize development in machines operating at varying speeds or having different size drums. It should also be clear to one skilled in the art that the rotating brush arrangement of the present invention is not restricted to use in conjunction with a cylindrical drum but can be readily adapted to be used with a Wide variety of endless belt photoconductor configurations as well as with moving flat plate photoconductors.

The brush cleaning arrangement, as herein disclosed, isolates the active development zone from the backside or return portion of the system thus giving the apparatus greater flexibility than afforded by the basic flow-contact system. The bottom wall of the active development zone is capable of acting much in the same manner as a biased electrode in conventional development systems when a DC potential is applied thereto and may be biased with a sufiicient DC potential to suppress background development and also to further enhance the solid area coverage capability of the development apparatus. For further information concerning the operation of development electrodes, reference is had to Walkup, U.S. Pat. No. 2,573,881; and Landrig-an et al., U.S. Pat No. 2,725,304.

Suflicient agitation of developer material to produce effective developer mixing and triboelectrification has been found to occur over the entire operating range of the present apparatus. That is, the present system is capable of delivering properly mixed and charged developer material to the entrance of the active development zone regardless of the flow rate being maintained in the system. The system is not dependent on the drum-developer interface flow for developer mixing but rather upon the agitation produced by the continuously rotating brush member. The brush member serves a second function other than cleaning residual toner particles and mixing developer material in that the brush member contacts an extremely high percentage of particulate developer material prior to the materials delivery to the active development zone to bleed off any space charge build up which might occur about the carrier material.

While this invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.

What is claimed is:

1. In an improved xerographic process comprising uniformly charging a moving photoconductive surface, exposing the charged surface to the pattern of light and shadow to form a latent electrostatic image thereon, moving the image bearing surface in an uphill direction in a generally ascending arcuate path, said arcuate path extending upwardly from the point of tangency of a tangentially horizontal imaginary line, contacting only said photoconductive surface as said surface is transported in the ascending arcuate path along a substantially continuous contact zone having an upper extremity and a lower extremity with a bath of developer material made up of toner particles and carrier beads triboelectrically arranged and in contact With the upwardly moving surface through a zone of active development of sufiicient length to efi'ect image development, the improvement comprising transferring the developed image onto a support material, recharging the photoconductive surface, re-exposing the charged photoconductive surface, and then brushing the residual toner particles from the photoconductive surface at the commencement of the development zone and then flicking the particles to introduce residual toner particles into a continuous supply of developer material to the active development zone to support the flow of material therethrough.

2. A process according to claim 1 including a step of supplying a pre-clean charge subsequent to the transfer toner particles and continuing all of the aforementioned steps in repeated cycles.

References Cited UNITED STATES PATENTS 3,503,776 3/1970 Gundlac'h 11717.5

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

